Many advanced direct methanol fuel cell systems use as the electrolyte a proton-exchange membrane made of perfluorocarbon material available from E. I. duPont, Wilmington, Del., USA, under the trade designation NAFION 117. However, when NAFION based membranes are used in direct methanol fuel cell applications, the methanol diffusion through the membranes is too high, which allows some fuel to cross over to the cathode side of the cell. Methanol crossover to the cathode results in fuel loss, lower overall efficiency and lower cell performance. Thus, a large amount of research has focused upon methods to decrease the methanol crossover rates in direct methanol fuel cells.
In addition to problems of methanol crossover, large water permeability is observed with NAFION based systems, which causes water management problems in fuel cell stacks. To address these issues, an alternative polymer electrolyte membrane has been proposed that is made of a composite of polystyrene sulfonic acid and poly(vinylidene difluoride). Initial studies of PSSA-PVDF membranes have shown lower methanol permeability than NAFION 117 membranes and comparable proton conductivity.
Conventional PSSA-PVDF membrane MEAs have been prepared using the same processes used to prepare NAFION membrane MEAs. Catalyst electrodes are prepared by making a catalyst ink including a catalyst material, typically either Pt or Pt/Ru, and an alcohol solution of a perfluorosulfonic acid ionomer available from E. I. duPont de Nemours, Wilmington, Del., USA under the trade designation Naflon-H. The catalyst ink was either applied to a substrate such as porous carbon paper or, alternatively, directly deposited upon the membrane surface. This sandwich structure was then placed in a hot press for bonding with catalyzed TEFLON impregnated porous carbon electrodes to form a single MEA component. While this method worked well for MEAs with NAFION membranes, MEA samples made with the PSSA-PVDF membranes made by this process displayed substantially lower electrical performance in experimental fuel cells and were generally characterized by high cell resistance and low catalyst utilization values.
Therefore, the electrical performance of MEAs with PSSA-PVDF membranes requires further improvement for implementation in power sources.